Along with construction of large capacity optical communication networks showing rapid development in recent years, the Wavelength Division Multiplexing (WDM) communication technology is attracting attention, with a larger number of facilities being built therefor, and it is common in a WDM node to convert optical signals into electric signals once without directly controlling the optical signals and subsequently perform path switching.
However, there are concerns with the aforementioned method, such as overloaded processing capacity, communication speed limitation, and high power consumption in the node. Accordingly, there is increasing importance of transparent network systems represented by ROADM (Reconfigurable Optical Add/Drop Multiplexer) configured to perform signal processing on optical signals directly as the optical signals without via electric switching, and therefore intensive development of optical devices constituting a ROADM such as a Wavelength Selective Switch (WSS) or a Tunable Optical Dispersion Compensator (TODC) is under way.
A general configuration and an operation principle for an optical signal processing device such as a WSS and a TODC will be described. A WDM signal input via an input optical fiber is propagated by a collimator through a space as collimated light and focused again via a lens after having passed through a plurality of lenses and a diffraction grating configured to perform wavelength demultiplexing. A Spatial Light Modulator (SLM) configured to provide the optical signal with a desired phase variation is arranged at the focusing position.
As a typical example of the aforementioned SLM, there may be mentioned a micro mirror array, a liquid crystal cell array, a DMD (Digital mirror device), an LCOS (Liquid crystal on silicon) or the like according to the MEMS (Micro-electro mechanical system) technology, by which each of the optical signals is provided with a desired phase variation and reflected. Each of the reflected optical signals is incident on a diffraction grating via a lens, wavelength multiplexed, and subsequently coupled to an output fiber via a lens.
When using an optical signal processing device as a compensation device represented by a TODC, there is frequently used a method that separates the signals before and after compensation using a circulator, unifying the input fiber and the output fiber. In addition, a switching device such as a WSS may be configured to have at least one input fiber and also a plurality of output fibers so as to cause the signal light incident from the input fiber to be output from an output fiber selected via the SLM. Deflecting the signal lights at desired angles by the SLM allows selecting the output fiber in which the reflected signal lights are coupled and switching.
With regard to a WDM node, there is disclosed a form of simultaneously implementing a plurality of such optical signal processing devices as described above (see Non Patent Literature 1).
FIG. 8 is a diagram illustrating a configuration of an optical signal processing device having a plurality of the conventional WSSs implemented in a single node. For an optical signal which has entered an optical node, a WSS group 801 sets a drop- or a through-path in a wavelength selective manner. For each of optical signals dropped by the WSS group 801, a wavelength demultiplexing function unit group 802 determines a path according to the wavelength, and the optical signal enters a receiver group 803 to reach a desired receiver. On the other hand, optical signals transmitted from a transmitter group 804 in the optical node pass through a wavelength multiplexing function unit group 805 and are transferred by a WSS group 806 to adjacent nodes.
In such a form, the wavelength and the route are determined (Colored/Directed) in accordance with the position of a port which inputs an optical signal to an optical node or receives an optical signal from an optical node. Accordingly, there are proposed various forms such as an approach (Colorless) which allows transmission and reception of signals of any wavelength by replacing the wavelength demultiplexing function unit group 802 and the wavelength multiplexing function unit group 805 with a WSS group in order to provide a more flexible functionality, or an approach (Directionless) which allows transmission and reception of signals from any route by inserting a matrix switch group respectively between the wavelength demultiplexing function unit group 802 and the receiver group 803, and between the transmitter group 804 and the wavelength multiplexing function unit group 805.
Here, it is often the case in either form that, with a combination of an add-WSS and a drop-WSS being as a single set, there are needed as many WSS sets as the number of routes in a wavelength cross connect function unit 807 illustrated in FIG. 8. Accordingly, the low cost N-in-1 WSS having two or more sets of WSS functions integrated in a single device is very attractive due to many merits such as suppressed initial introduction cost, reduced power consumption, and reduced load on the control system. It goes without saying that functions to be integrated in a single device is not limited to N sets of WSS functions, and different functional forms such as WSS and TODC or the like may also exhibit a high effect.
FIGS. 9A and 9B illustrate a conceptual diagram of a general configuration of an optical signal processing device having a plurality of functions integrated therein. The functions to be integrated in the optical signal processing device are assumed to be two sets of WSS functions, with the direction of wavelength demultiplexing by the diffraction grating defined as the x-axis, the traveling direction when an optical signal is output from the fiber defined as the z-axis, and the direction perpendicular to the x-axis and the z-axis defined as the y-axis. In addition, although it is assumed for ease of explanation that there is one input port and two output ports in one WSS, but the number and configuration are not limited to those in the following description (see Non Patent Literature 2). In addition, the chief ray of an optical signal output from the first WSS function unit is indicated by a solid line, and the chief ray output from the second WSS function unit is indicated by a dashed line, respectively.
Firstly, the configuration of FIG. 9A will be described. Input and output of optical signals are performed via an input/output port group 901, and the input/output port group 901 can be divided into a first input/output port group 901-1 corresponding to the first WSS function unit and a second input/output port group 901-2 corresponding to the second WSS function unit. In the configuration in FIG. 9A, the first input/output port group 901-1 is exemplified as the lower three ports in FIG. 9A, and the second input/output port group 901-2 is exemplified as the upper three ports. The traveling directions of the optical signals respectively output from the first input/output port group 901-1 and the second input/output port group 901-2 are all parallel, and correspond to the z-axis in the present example.
The optical signal output from the input/output port group 901 to the space propagates while spreading by a certain Numerical aperture (NA) according to the diameter of the beam which has been trapped in the port. Generally, the input/output port group 901 is often implemented by a combination of an optical fiber array and the microlens array 902 so that the optical signal output from the port turns into collimated light. The optical signal that has propagated through the space is Fourier transformed and the position/angle transformed by a lens group 903 provided in a distributed manner for each WSS function unit. Subsequently, the optical signal enters a diffraction grating 905 at a predetermined angle for each WSS function unit via a lens 904 to be wavelength-demultiplexed in the x-axis direction, and further is focused onto a spatial light modulator 907 via a lens 906.
The spatial light modulator 907 has a beam deflection function, whereby it becomes possible to switch the output ports by appropriately controlling the deflection angle. On this occasion, optical design is made such that optical axes associated with the first input/output port group 901-1 intersect at a single point located at the upper part of the spatial light modulator 907 in FIG. 9A, and optical axes associated with the second input/output port group 901-2 intersect at a single point located at the lower part thereof. In other words, the optical signals focused on the spatial light modulator 907 will be independently focused on different positions in the y-axis direction for each WSS function unit. Setting the deflection angle independently for each WSS function unit using the spatial light modulator 907 can realize two sets of WSS functions in a single device.
The configuration of optical signal processing device having integrated therein a plurality of functions is not limited to that illustrated in FIG. 9A. FIG. 9B is an exemplary configuration in which the lens group 903 and the lens 904 in the configuration of FIG. 9A are omitted. Also in the example, the traveling directions of the optical signals output respectively from both of the first input/output port group 901-1 and the second input/output port group 901-2 are parallel, and correspond to the z-axis in the present example. In order to focus light at different positions on the spatial light modulator 907 for each WSS function unit, the lens 906 previously having a single-lens configuration is turned into a two-lens configuration with each lens being separately arranged for each function unit, whereby integration of a plurality of functions can be realized.